If you’re a Keynote user, you probably know you can group slides together, which is a great way to keep the various pieces of your presentation separated. To create a group, you just decide which slide you’d like to have as the first slide in the group—typically, this would be the slide that introduces that particular section of your talk. You then select any number of following slides, and either drag-and-drop them onto this first slide, or just press Tab (Shift-Tab to ungroup).
This is one of Keynote’s nicer features, especially if you’re working on, for instance, a 90-minute-long “Best of Mac OS X Hints” presentation for Macworld Expo—instead of dealing with perhaps a hundred different slides, I can deal with maybe 10 groups of 10 slides each, expanding only the group I’m currently working with.
There’s a semi-hidden bonus in this grouping feature, too—a simple way to copy multiple slides from one presentation to another. Just collapse the group of slides you’d like to copy, press Command-C to copy them, switch to your other presentation, and press Command-V. Just like that, all the slides in the group will be pasted into the presentation.
Of course, the flip side of this bonus is that if you only want to copy the first slide of the group—perhaps as a shortcut in creating the section header for another section of your presentation—you’ll need to expand the group before copying the first slide, otherwise you’ll get the whole group. Just keep this in mind when duplicating slides, lest you be surprised by the unexpected appearance of 25 slides when you hit paste!